It is well known in the art to form thermoplastic containers by injection formation of a closed end preform followed by a blowing of the preform to yield the desired container. See, for example, U.S. Pat. Nos. 3,850,562 and 4,151,247.
The apparatus and process described in U.S. Pat. No. 3,850,562 achieves the injection formation of the preform with the use of an unitary injection mold into which a core pin is inserted. Into the preform cavity, which is defined by the space between the core pin and the injection mold cavity, there is injected the thermoplastic material of choice. After this material has cooled sufficiently, the core pin is removed and the preform is stripped from the injection mold and sent to the blow molding portion of the apparatus. The preform, as it is being stripped from the injection mold, will undergo forces tending to distort or damage it. To enable the preform to withstand these forces without distortion, it must have a sufficient "set". The degree of "set" is dependent upon the extent to which the preform is cooled. Cooling is usually achieved by passing a cooling fluid through channels in the injection mold, and in some cases the core pin. Since cooling is achieved by heat transfer to the cooling fluid it is time dependent. It has been found that when using a unitary injection mold that the cooling time required to obtain sufficient "set" for deformation-free stripping of the preform from the injection mold exceeds the cooling time necessary when utilizing a split injection mold such as the one described in U.S. Pat. No. 4,151,247. By using a split injection mold, the degree of preform "set" is less than is necessary for a unitary injection mold due to the fact that the preform is removed from the split mold by having the molds open and move away from the preform and thus the forces on the preform are considerably less. Since the cooling time is lowered by using split injection molds, a shorter overall cycle time for the apparatus is required.
Despite this clear advantage, injection-blow molding apparatuses which utilize split injection molds have one serious drawback, i.e., the alignment of the split injection mold with the core pin must be assured with great exactitude so that when the molds come together to form the injection mold cavity, the core pin will be in exact alignment therewith. Such alignment is very time consuming and thus expensive. If there is not exact alignment, the core pin, when lowered into the cavity, will not be centered and the resultant preform will have variant wall thicknesses. Non-uniformity in preform wall thickness results in poor quality containers as the container will have thinned-out wall portions which represent weak spots in the containers.
The time penalty for achieving alignment is exacerbated if the molder has to achieve this alignment every time he has to change injection molds to fit new customer requirements. This extra time penalty could be avoided if the molder could achieve a single initial alignment and then merely change out customer moldings with each new customer mold taking its alignment cue from the initial alignment. Thus, only one alignment, i.e., the initial alignment, need be made irrespective of the number of mold changes required.
Therefore, it is an object of this invention to provide a system for use on a split mold injection-blow molding apparatus which requires only a single initial alignment irrespective of the number of times the split injection molds are changed out.